1. Field of the Invention
The present invention relates to -a throttle-valve control apparatus for allowing switching from an ordinary-combustion state to an AR-combustion state or vice versa to take place with ease in an AR-combustible spark-ignition two-cycle engine wherein, at least during a light-load operation, a compression-start pressure is properly controlled in order to let a fresh air/fuel mixture in a combustion chamber self-ignite so as to undergo active thermal atmosphere combustion.
2. Description of the Prior Art
In the conventional vehicle spark-ignition two-cycle engine mounted on a motor bicycle or the like, an exhaust port and a scavenging .port are created on the inner surface of a cylinder. The exhaust and scavenging ports are opened and closed by a piston. A fresh air/fuel mixture precompressed in a crank chamber is supplied into the cylinder through the scavenging port and, at the same time, burned gas is exhausted from the cylinder through the exhaust port. The fresh air/fuel mixture compressed in the cylinder is then ignited by spark plugs. Even during an idle operation, it is necessary to supply a fresh air/fuel mixture of a certain amount to the cylinder. Thus, a throttle valve on an air intake passage needs to be opened to a certain degree, in actuality, to a position greater than about 10% of the complete opening of the throttle valve.
On top of that, such a conventional spark-ignition two-cycle engine has another problem in that, if the exhaust port is enlarged for raising the output and efficiency in the region of high-speed and heavy-load operations to values greater than already high standards, the amount of unburned hydrocarbon in the region of light-load operation due to blow-by of a fresh air/fuel mixture and combustion instability increases, resulting in inefficient fuel consumption.
In order to solve the above problems, the applicants of the present invention designed an AR combustible spark-ignition two-cycle engine that can perform activated heat atmosphere combustion in the region of light-load operations through self-ignition with timing desirable for engine operations by properly narrowing the exhaust port or the exhaust passage in accordance with the current operational state so as to allow fresh air in the combustion chamber to be activated by thermal energy contained in burned gas of the previous cycle. The patent was applied for with Patent Application No. H5-187488.
The combustion wherein the ignition timing preferable for operation of an engine is controlled positively to cause activated heat atmosphere combustion to take place in this manner is hereinafter referred to as AR combustion. A detailed description of AR combustion can be found in publication "A Trial for Stabilizing Combustion in Two-Stroke Engines at Part Throttle Operation", by Yoichi Ishibashi and Yuji Tsushima, which is herein incorporated by reference.
An attempt to apply the throttle-valve control mechanism of the conventional spark-ignition two-cycle engine to the AR-combustible spark-ignition two-cycle engine, that can perform AR combustion as described above, in the region of light-load operations was found unsuitable to reduce the opening of the throttle valve to an amount proper for AR combustion without applying some operational force to an operation member of the throttle valve.
If the opening of the throttle valve is set at an amount proper to the AR combustion at a position with no operational force applied to the operation member of the throttle valve, the throttle valve must be sustained at a required amount of opening with some operational force applied to the operation member of the throttle valve at an idle-operation state resulting from ordinary spark ignition. In other words, an idle operation cannot be carried out with the operation member of the throttle valve released.